Electrokinetic transport (electroosmosis, electrophoresis) of chemical species through thin slabs or through narrow conduits is known in the art. However, more recently, new devices with eletrokinetic-transport elements have been disclosed. In particular, devices described in the literature have been directed towards applications of eletrokinetic-transport technology in genomics, proteomics, combinatorial chemistry and high-throughput screening for drug discovery.
Electrokinetic-transport technology is used in species separation devices including slab-gel electrophoresis devices. In the slab-gel electrophoresis method, separation of chemical species occurs when the species in aqueous solution are transported at different rates along the gel. This class of prior-art devices generally consists of macro-scale slabs of hydrophilic-gel materials. Examples are described in U.S. Pat. No's. 4,574,040 and 4,663,015. In systems such as those described in the above referenced patents, pouring a gel-forming liquid into the space between two glass plates forms the slabs. The gel-forming liquid is an aqueous solution of hydrophilic polymers and cross-linkers. The gelation process causes the liquid to solidify into a solid slab. The resultant gel slab is a solid matrix containing a substantial quantity of water. The slab thickness is determined by the spacing between the plates maintained by spacer strips placed between and along two opposing edges of the plates. Clamps hold the plates together and the spacer strips are smooth so that a seal is formed under the pressure of the clamps, preventing leakage of either the gel-forming solution during gel casting or the buffer solution during electrophoresis.
Methods disclosed to reduce the dimensions of the transport channel of the slab-gel devices have generally used macrofabrication techniques. For example, U.S. Pat. No. 5,627,022 discloses a thin gel slab prepared inside a gel holder consisting of two planar substrates and a thin spacer consisting of beads in an adhesive matrix. Microfabricated multiple separation lane slab-gel devices have been disclosed, for example in U.S. Pat. No. 5,543,023. Such devices consist of an array of thin slabs separated by spacers. Multiple lane devices with gels cast into microchannel arrays are known in the art. U.S. Pat. No. 5,192,412 discloses a slab-gel contained within plates wherein one plate has a linear array of microchannels. U.S. Pat. No. 5,746,901 discloses a similar combination of corrugated and flat glass plates sandwiching gel slabs. U.S. Pat. No. 5,954,931 discloses an electrophoresis device with parallel channels formed by casting gel onto a substrate with microchannels. Gel compositions for small dimension electrophoresis gel slabs have been disclosed in U.S. Pat. No. 6,013,166. Macro-scale dried gel slabs that are reconstituted by treatment with water prior to use have also been reported in the prior art (U.S. Pat. Nos. 4,048,377 and 4,999,340).
Species separation devices of the prior art also include capillary tubes used both for capillary electrophoresis and capillary chromatography (for example U.S. Pat. No. 5,207,886). In this technique separations are conducted by electrokinetic flow of liquid through narrow-bore glass capillary tubes. In these prior-art capillary devices the separation occurs within the capillary tube and the separation medium is a liquid that fills the tube after it is introduced through one end of the tube. Some prior-art devices use a polymeric coating on the internal surface of the narrow-bore tube (U.S. Pat. Nos. 5,141,612 and 5,167,783), others use capillary tubes pre-filled with gel (U.S. Pat. No. 4,997,537), still others introduce the separation polymer dissolved in the sample liquid (U.S. Pat. No. 5,089,111).
Multi-lane separation devices consisting of multiple capillary tubes assembled in a housing have been disclosed in the art, for example U.S. Pat. No. 5,439,578. It is well known in the art that such capillary separation devices provide superior separation performance over slab-gel separation devices of the prior art because narrow bores provide for less spreading of the species in the separating medium. Also, because of superior heat dissipation, high voltages can be used to effect rapid separation.
Some shortcomings of these devices include the inability to easily integrate with other fluid manipulation elements or other elements of the analytical process and the inability to provide readily for variations of composition within the medium.
Integrated micro-analytical and micro-chemical-reaction devices, commonly also referred to as lab-on-a-chip devices, have been disclosed in the prior art (for example U.S. Pat. Nos. 4,908,112 5,126,022 and 5,180,480). These devices utilize micro-machining methods adapted from semiconductor chip manufacturing to fabricate micro or meso-scale devices on planar substrates for the purpose of performing separations, measurements and chemical reactions. These devices are mechanical structures realized by forming cavities and channels or trenches into a solid substrate. The devices are generally completed when a cover assembly over the cavitated substrate provides a cap that converts the cavities and channels into chambers and conduits. U.S. Pat. No. 5,429,734 however, discloses a channel etched into a semiconductor wafer that includes a monolithic capping means. U.S. Pat. No. 4,908,112 discloses separation devices including electrodes with channels etched into semiconductor slabs. U.S. Pat. No. 5,750,015 discloses separation devices with trenches formed in insulating plastic slabs. Other structures consisting of cavitation in planar substrates include devices with channels and detectors (U.S. Pat. Nos. 5,637,469 and 5,906,723), devices with chambers (U.S. Pat. No. 5,585,069) and devices with channels and mechanical sieving means (U.S. Pat. No. 5,304,487). Reactions, mixture separations and analyses take place in such microstructures in liquids that are electrostatically transported along the conduits. Generally in these prior art devices, the reactants, catalysts and reagents are stored and prepared in off-chip processes then introduced into the channels of the chip during use by pumping from one open end of the channel along its entire length. U.S. Pat. No. 5,126,022 discloses microfabricated trenches that are filled with gel prior to use.
Integrated micro-channel separation devices have been disclosed in the art, wherein electrokinetic fluidic manipulations are carried out in micro-channel structures more complicated than those feasible in a simple capillary tube with only an inlet and an outlet, and more complicated than an array of channels either in multi-lane slabs or capillary tube arrays. U.S. Pat. No 5,770,029 discloses a device with a main electrophoretic channel connected to a secondary, enrichment channel. U.S. Pat. No. 5,296,114 discloses an electrophoretic separating device consisting of a channel in the form of a loop with multiple inlet and outlet ports. U.S. Pat. No. 5,750,015 discloses a device consisting of a main trench and multiple branching trenches. Devices are disclosed with multiple connected channels (U.S. Pat. No. 5,800,690), intersecting channels (U.S. Pat. Nos. 5,599,432 and 6,010,608) and channels connected to multiple reservoirs (U.S. Pat. No. 5,858,195).
Integrated micro-channel devices in which there is a binding step combined with an electrokinetic transport step within a conduit or slab are also known in the art. U.S. Pat. No. 5,661,028 discloses a device that integrates a binding/primer element with an element for introducing reagents from off-chip for a Sanger sequencing reaction with an electrophoretic separation element consisting of a planar etched channel with glass cover plate backfilled with gel. U.S. Pat. Nos. 4,628,035 and 5,055,415 disclose antigen-antibody binding inside an electrophoretic medium.
The prior art of biosensors and dry reagent diagnostic devices contains numerous uses of hydrophilic materials or gels. Devices from this prior art that are made by microfabrication include for example U.S. Pat. No. 5,194,133 that discloses a biosensor with a micromachined channel filled with a gel material. Devices that consist of a composite of a gas-permeable layer and a hydrophilic-polymer layer also are known in the prior art of biosensors, including devices of this type made by microfabrication. For example U.S. Pat. No. 4,933,048 discloses a microfabricated gel and hydrophobic-vapor-permeable polymer for use as a salt bridge of a potentiometric reference electrode. U.S. Pat. Nos. 5,514,253 and 5,200,051 disclose micro-fabricated gas and enzyme biosensors that also utilize these composite layers. These numerous diagnostic devices disclosed in the prior art of biosensors utilize the gel or hydrophilic material as a medium for reagent retention or as an element through which species move by diffusion. However, none of these references teach the use of a layer composite of this general type in an active electrokinetic pumping application. Both the functional design and the mode of operation of this class of prior-art biosensor and dry-reagent diagnostic devices are different from active electrokinetic pumping devices.
Devices have been disclosed in the prior art that utilize voltages not for electrokinetic transport but for modulating the amount of hybridization at an electrode surface (U.S. Pat. Nos. 5,632,957 and 6,017,696) or for biological sample preparation (U.S. Pat. No. 6,129,828).
In summary, prior-art electrokinetic devices are either empty channels (trenches in planar substrates or conduits in tubes), channels with coated surfaces, or channels filled with polymer solutions or gel. Prior-art devices also include slabs of gels or gel tracks formed by casting gels into mechanical pre-forms or cavities. The prior-art devices are thus limited in one of several ways. Prior-art micro-channel devices, while manufactured in part by microfabrication methodologies, generally only provide for elements that contain mechanical structures. Thus they do not contain the chemicals and reagents required to function as truly integrated-analytical systems. At the current state of the art these types of devices consist of really only lab-glassware-on-a-chip rather than the complete lab-on-a-chip as they have been called. The prior art does not teach how the integration of chemical function can be accomplished with any generality. Furthermore, prior-art slab-gel based devices are generally made by traditional macro-fabrication methods, thus they are expensive to manufacture and use. They require large sample sizes and are slow in performance. They cannot easily be integrated either to provide multi-analysis capability, nor easily or cost effectively be combined with other components of an integrated analytical system.
Moreover, the materials of the transport element of prior-art slab-gel devices have been limited to gelatinous media. As such they are largely water-based and fragile and difficult to process into structures much more complicated than simple slabs. These materials are not amenable to planar processing nor microfabrication to make integrated devices. Thus there remains a significant need for cost-effective electrokinetic devices amenable to planar processing and/or microfabrication and for processes for their manufacture. A further need exists for electrokinetic devices with incorporated chemical entities.